Flow allocation in drill bits

ABSTRACT

A method for designing a drill bit comprising modeling a domain between a drill bit and a surrounding wellbore. A region is defined within each of a plurality of flow paths through which fluid travels through the domain. An allocation of flow among the plurality of flow paths through the domain is determined and the drill bit is modified such that the allocation of flow is substantially uniform among the plurality of flow paths. A drill bit comprises a bit body having a plurality of blades projecting there from, wherein at least one blade has a greater length than at least one other blade. A plurality of nozzles, or ports, are disposed on the body and a plurality of junk slots are formed between adjacent blades so that the flow of fluid through each of the plurality of junk slots is substantially uniform.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of, and incorporates byreference, provisional application Ser. No. 60/618,060, filed Oct. 12,2004, and entitled “Flow Allocation in Drill Bits.”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND

The present invention relates generally to earth boring drill bits. Moreparticularly, the present invention relates to methods and apparatusused to allocate and control fluid flow through and around earth boringdrill bits. Still more particularly, the present invention relates tomethods that use hydraulic analysis to determine the fluid flow throughand around earth boring drill bits and apparatus that provide for theadjustment or variation of certain drill bit parameters in order toallocate and control the fluid flow.

In rotary drilling applications, an earth boring drill bit is disposedat the end of a rotating drill string. A fluid is pumped down throughthe drill string to the bit, where it exits the bit through one or morenozzles or ports. The interaction of the fluid with the drill bit andthe surrounding formation is an important aspect of drill bit design andperformance. This system of interaction is known as “bit hydraulics.”Evaluation of bit hydraulics generally comprises analyzing three primaryfunctions of the hydraulics, namely: cutting structure cleaning/cooling;bottom hole cleaning; and cuttings evacuation.

Cutting structure cleaning is the ability of the hydraulic fluid toremove formation materials from the bit's cutting structure.Accumulation of formation materials on the cutting structure can reduceor prevent the penetration of the cutting structure into the formation.Cutting structure cooling is the ability of the hydraulic fluid toremove heat, which is caused by contact with formation, from cuttingelements in order to prolong cutting element life. Bottom hole cleaningis the ability of the hydraulic fluid to remove cut formation materialsfrom the bottom of the hole. Failure to remove formation materials fromthe bottom of the hole can result in subsequent passes by cuttingstructure to re-cut the same materials, thus reducing cutting rate andpotentially increasing wear on the cutting surfaces. Cuttings evacuationis the ability of the hydraulic fluid to move cut formation particlesaway from the area immediately surrounding the drill bit. Failure tocirculate formation cuttings up the annulus and away from the drill bitcan also lead to reduced penetration rates and premature wear of cuttingsurfaces.

The three functions of bit hydraulics should be properly addressedthrough bit hydraulic design to provide for best overall bitperformance. However, because each drilling situation may besignificantly or slightly different depending on many factors, carefulconsideration should be paid to the bit hydraulic system design. Thedrilling situation depends on factor that include, but are not limitedto, the bottom hole assembly, drilling fluid type, rig capability,formation type, drilling rate, and drilling depth.

Also playing an important role in this bit hydraulics system design isthe style or method by which fluid is discharged from the bit. Commonly,a nozzle receptacle receives an erosion resistant, replaceable nozzlethrough which fluid is discharged. This receptacle oftentimes is anintegral feature of the bit made during the manufacturing process andoffers means for orientation and retention to the separate nozzle partwhen installed. Common means of nozzle retention include by screwthread, snap-ring, or nail, however other means do exist.

Nozzle selection is an important step in designing a bit hydraulicssystem. Fundamental selection aspects include the nozzle orificediameter and nozzle design. Nozzle orifice diameter, or nozzle size,directly relates to the nozzle's ability to restrict flow and createdesired pressure loss. In addition to diameter, the total-flow area(TFA) of a nozzle can be used as a basis for nozzle size selection andcan be determined by calculating the cross sectional area of the nozzleat its exit. In cases of difficult to measure geometry, TFA can bedetermined experimentally and presented as an equivalent-TFA relative tosome known situation. In general, increasing orifice diameter or TFA ofa nozzle can result in a higher efficiency nozzle having less fluidrestriction and a smaller magnitude pressure loss for a given flowrate.

Another element for nozzle selection is nozzle design. Some nozzles aredesigned to discharge fluid streams with very little jet expansionresulting in more concentrated and efficient energy delivery whereasother designs, such as diffusers, encourage diffusion and mixing andstill others significantly redirect the discharging fluid stream. Thelarge number of nozzle designs available exists to facilitate adjustingbit performance in the various different drilling applications andsituations.

As an alternative to replaceable nozzles, the discharge location maycomprise a nozzle port, which is a fluid passageway formed between theinternal portion of the bit and the bit exterior. Ports generally do notallow the end-user flexibility to adjust its configuration. In mostinstances, the port's configuration is adjusted by modifying the passagegeometry and is implemented in manufacturing. Similarly, as withreplaceable nozzles, the larger TFA ports are less restrictive and thusproduce lower magnitude pressure losses. Also, as with replaceablenozzles, various port designs are available for a large variety ofintended drilling applications.

There are two predominate types of rock bits, namely roller cone rockbits and rotary drag bits. Commonly, drag bits are referred to aspolycrystalline diamond cutter (PDC) bits since cutters containpolycrystalline diamond on the cutting surface. Drag bits are oftencharacterized by cutters grouped and placed on several blades. Many dragbit designs include primary blades, secondary blades, and sometimes eventertiary blades, where the primary blades are generally longer and startat locations closer to the bit's rotating axis. The blades projectradially outward from the bit body and form flow channels therebetween.Drag bits also include nozzles or fixed ports that serve to inject fluidinto these flow passageways. As fluid is injected from the nozzles andflows through the flow channels, the fluid removes cuttings and cleansthe cutting structure. The fluid carries the cuttings through the flowchannels and upwards into the annulus through the passageways formed bythe blades of the drill bit and the surrounding hole, which are commonlyknown as “junk slots.” The movement of fluid through the junk slots isan important factor in the performance of the drill bit.

Thus, there remains a need to develop methods and apparatus that provideimproved bit hydraulic performance by providing for an evaluation theallocation of flow across the bit as well as bit design features thatallow for adjusting and controlling the allocation of flow in order toovercome some of the foregoing difficulties while providing moreadvantageous overall results.

SUMMARY OF THE PREFERRED EMBODIMENTS

The preferred embodiments include methods for designing drill bitshaving bit parameters that provide a substantially balanced flow among aplurality of flow paths. A method for designing a drill bit comprisesmodeling a domain between a drill bit and a surrounding wellbore. Aregion is defined within each of a plurality of flow paths through whichfluid travels through the domain. An allocation of flow among theplurality of flow paths through the domain is determined and the drillbit is modified such that the allocation of flow is substantiallyuniform among the plurality of flow paths. A drill bit comprises a bitbody having a plurality of blades projecting there from, wherein atleast one blade has a greater length than at least one other blade. Aplurality of nozzles, or ports, are disposed on the bit and a pluralityof junk slots are formed between adjacent blades so that the flow offluid through each of the plurality of junk slots is substantiallyuniform.

Thus, the present invention comprises a combination of features andadvantages that enable it to overcome various problems of prior devices.The various characteristics described above, as well as other features,will be readily apparent to those skilled in the art upon reading thefollowing detailed description of the preferred embodiments of theinvention, and by referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the preferred embodiment of thepresent invention, reference will now be made to the accompanyingdrawings, wherein:

FIG. 1 is a bottom view of a drag bit;

FIG. 2 is a flowchart representing a method for designing a drill bithave a substantially uniform allocation of flow;

FIG. 3 illustrates the flow areas through the junk slots of a drag bit;

FIG. 4 is a graph representing the allocation of flow through the junkslots of a drag bit;

FIG. 5 illustrates one embodiment of a drag bit having multiple nozzlesin a single junk slot;

FIG. 6 illustrates nozzle parameters for a single nozzle of a drag bit;

FIG. 7 illustrates one embodiment of a drag bit having nozzles withadjusted nozzle parameters;

FIG. 8 illustrates a nozzle having a shallow seat depth;

FIG. 9 illustrates a nozzle having an increased seat depths;

FIG. 10 illustrates a drag bit having nozzles with adjusted seat depths;

FIG. 11 illustrates a drag bit with increased blade fill; and

FIG. 12 is a visual representation of the allocation of flow through thejunk slots of two different bit designs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one class of embodiments, the present invention includes methods andapparatus that allow a drill bit to be designed, analyzed, andconstructed such that flow is substantially uniform among several flowpaths that carry fluid away from the drill bit. FIG. 1 illustrates adrag bit 10 comprises a bit body 12 having three primary blades 14,three secondary blades 16, and six nozzles 18. Primary blades 14 have agreater length than secondary blades 16 and extend closer to the bit'scentral axis. Junk slots 19 are formed around the circumference of bitbody 12 between blades 14, 16. Nozzles 18 are disposed on bit 10generally between blades 14, 16. In operation, drilling fluid flowsthrough nozzles 18, past blades 14 and 16, and through junk slots 19 asit moves up the annulus toward the surface. As discussed above, thedrilling fluid acts to cool the cutting structures and to removecuttings from the blades as well as from the bottom of the hole as thehole is being drilled.

Analysis and experience has shown that the distribution or allocation offluid through the junk slots is not easily predictable. For instance, ina conventional six-bladed bit having six, symmetrically arranged blades,junk slots, and nozzles, one would think each junk slot would receive anamount of fluid equivalent to the other slots (i.e. 16.7% of the totalflow through the annulus). However, it has been learned that the fluiddistribution for any junk slot could easily have values of 25%, 10%, or5% due to the interaction between the fluid, bit geometry, and nozzleconfiguration.

Therefore, embodiments of the present invention include methods thatallow a drill bit to be designed, analyzed, and constructed such thatflow is substantially uniformly allocated among the several flow pathsthat carry fluid away from the drill bit. Embodiments of the presentinvention include methods that allow a designer to use computationalfluid dynamics (CFD) to evaluate drill bit performance. Once a drill bitdesign is chosen, that design can then be analyzed using CFD todetermine the bit's hydraulic performance, including the allocation offlow among several flow paths. The analysis of the hydraulic performanceof the bit can be performed as a set of fluid flow and other conditionsthat are based on identified drilling parameters or other criteria. CFDanalysis tools allow detailed analysis of the fluid flow field throughand around the drill bit structure and bottom hole region. CFD analysiscan provide data that can be used to generate information as to thefluid velocity, pressure, turbulence, direction, temperature, and otherfluid characteristics within the modeled volume.

To analyze a drill bit using CFD, a model is generally designed in a CADsystem or other applicable software which creates a set of boundingsurfaces which encapsulate the fluidic area of interest. Typically thebounding surfaces, known as the physical domain, will comprise the drillbit and an area representing the drill string, the bore hole that isbeing drilled, the hole bottom, and an exiting surface up the bore hole,which can be represented by an annular ring between the borehole and thedrill string. Once the bounding surface model is complete, a mesh, whichmay be constructed of various element types, is created in meshingsoftware. This mesh is called the computational domain, or domain, andis used by the CFD solver to calculate a solution comprising the fluidicproperties at each element, or cell, within the domain. Once the solverhas completed a solution, fluidic properties can be determined at anylocation within the confines of the domain. In order to betterunderstand the allocation of flow through the domain, methods ofanalysis and design processes using regions defined within the domainhas been developed as a part of certain embodiments of the currentinvention.

The following discussion uses a drag bit as an example but theprinciples and methods discussed herein are equally applicable to othertypes of bits, including roller cone bits. For example, in analyzingroller cone bits, the designer may want to have substantially uniformflow in the annular space above the gage, which is the outermost surfaceof the drill bit. In this case, instead of looking at flow allocation injunk slots, one may examine flow allocations in certain angularintervals around the bit.

As discussed above, certain drill bit designs utilize fixed ports fordischarging fluid into the wellbore. In the case when both fixed portsand replaceable nozzles are used on a bit, the adjustment of fluiddischarge sizes, orientations and locations can involve both port andnozzle configurations. When the term nozzle is used herein it isunderstood that it refers to a replaceable nozzle, a fixed outlet port,and any other location from which fluid is discharged from the drillbit.

Referring now to FIG. 2, one such design process 20 utilizes CFD tosimulate the flow within a domain. In this method, the first stepcomprises selecting a drill bit design 22 that may comprise a hydraulicconfiguration including one or more bit design parameters. In general,bit design parameters are defined as any aspect of the nozzle or portparameters and/or bit geometry parameters that can be changed so as toinfluence the behavior of the fluid as it circulates through and aroundthe drill bit. Examples of nozzle or port parameters include, but arenot limited to, quantity, design, size, location, angular orientation,seat depth, and arrangement. Examples of bit geometry parametersinclude, but are not limited to blade height, width, and length, bladeshape, bit body geometry, bit interior geometry, and number of blades.Other bit geometry parameters may also include, profile, height, andwidth of blade and blade-fill, transition between body and blades,shape, height, and width of junk slots and waterway, and side-rake,back-rake, location and orientation of cutters.

Once the drill bit design is selected, the domain between the drill bitand the surrounding wellbore is modeled 24. Modeling the domain mayfurther comprise constructing a computerized CAD model of a portion ofthe drill bit and wellbore, generating a mesh, which is also called thecomputational domain or domain, to represent the space between the drillbit and the surrounding wellbore, and establishing boundary conditionsfor the domain. A CFD solver is then utilized to simulate flow throughthe modeled domain and generate a CFD solution. The CFD solutioncomprises data representing the flow of fluid through the domainincluding the associated fluidic properties.

Once the CFD solution is complete, a plurality of regions is defined 26within the flow paths of fluid moving away from the bit. As an example,FIG. 3 illustrates a drag bit 32 disposed in a wellbore 34 where aplurality of regions 36A-F have been defined within the junk slots ofbit 32. Using the CFD solution, the flow allocation among the pluralityof regions can be determined. The flow through each region can berepresented by a volumetric flow rate, a mass flow rate, a fluidvelocity, or any other fluidic property that provides a representationof fluid flow behavior.

Once the plurality of regions is defined, the base fluidic propertiescan be found. For example, in one embodiment, each of the regionsco-planar surfaces defined within a junk slot. The fluidic property ofinterest for each region is the volumetric flow rate passing through thedefined surface. Generally, the volumetric flow rate through the regionis found by integrating the normal velocity over the area of thesurface. Depending on the direction of the velocity vector at each pointalong the surface, the fluid may be moving upward from the surface ordownward from the surface In this case, up and down do not indicate upand down relative to gravity but rather opposing directions relative tothe local surface normal direction. In one case, a designer may beinterested in the amount of fluid that is moving up and the amount offluid that is moving down. In another case, the designer may also beinterested in the volume of the fluid moving up versus the volume movingdown which could also be displayed in the form of a ratioQ_(up)/Q_(down). The designer may be further interested in looking atratios relative to the total flow moving through the plane, which may bedetermined as Q_(tot)=(abs(Q_(up))+abs(Q_(down))). Moreover, thedesigner may be interested in looking at the net flow rate of fluidthrough the region defined in the junk slot, which may be determined asQ_(net)=Q_(up)−Q_(down) In other embodiments, the fluidic property ofinterest may be the upward flowing velocity vectors and/or the downwardpointing velocity vectors.

Once the fluidic property data from the individual regions is collected,it can then be processed to determine the allocation of flow among thejunk slots or other flow paths. The allocation of flow can then bepresented as a visual representation, such as a graph, plot, contour, orother visual representation that is easy to understand and analyze.Referring back to FIG. 2, once the fluidic property and allocation offlow has been determined, it can be evaluated 28 by comparing it to apre-established design criterion (e.g. a maximum standard deviation) orby comparing it to results from a baseline configuration or other solvedhydraulic configurations with similar bit body and cutting structuredesigns in order to look for beneficial fluidic properties andallocation of flow.

In certain embodiments, it is desirable to provide a substantiallyuniform flow among the plurality of flow paths through the domain. As itis used herein, substantially uniform flow is where the standarddeviation of the fluidic property among the plurality of flow paths isless than 10%. Preferably, substantially uniform flow is where thestandard deviation is less than 5% and more preferably where thestandard deviation is less than 3%.

FIG. 4 shows results from computational fluid dynamics (CFD) simulationfor the flow distribution in the junk slots 36A-F. Junk slot 36A, 36C,and 36E have much stronger flow rates relative to the other junk slots.The standard deviation 38 of the percentage of flow rates is 11% andindicates a higher than desired non-uniformity of flow through thedifferent junk slots 36A-F. The high non-uniform flow rates through thejunk slots may cause undesirable fluid circulation above the drill bit.The junk slots with lower flow rates (36B, D, F) will typically have alarger degree of fluid circulation within the junk slots. Thisintra-slot circulation may form loops such that cuttings carried awaythrough the high flow rate junk slots from the hole-bottom arecirculated back to the hole-bottom through the junk slots with lowerflow rates.

Referring back to FIG. 2, based on the evaluation of the flowallocation, the drill bit design can then be modified 30. Once the drillbit design is modified, the method shown in FIG. 2 can be run again witha modified domain being modeled between the new drill bit design and thesurrounding wellbore. Again, a plurality of regions is defined in thesame locations to evaluate the flow allocation. This process is repeateduntil the evaluation 28 of the series of simulations indicate apreferential interior surface parameter, or set of parameters, for thefinal design. In this manner, the modified drill bit design can beevaluated to determine if the changes made improved the flow allocationand drill bit performance.

There are several approaches to improve the uniformity of the flowdistribution in the junk slots. These approaches can include, but arenot limited to, adjusting one or a combination of bit design parameterscomprising nozzle parameters and/or bit geometry parameters. Because theselection and installation of nozzles is often undertaken in the field,in certain embodiments, it may be desired to achieve improved uniformityof flow distribution by modifying bit parameters independent of nozzlesize. For example, in certain embodiments, substantially uniform flowmay be achieved in a multiple blade drag bit that has one nozzle perblade by configuring the nozzles such that the radial locations, nozzleseat depth, nozzle skew and profile angles are substantially the sameand modifying the blade geometry, such as blade length or thickness,including where at least one of the blades does not extend to bitcenter.

In other embodiments, a bit design may have a plurality of primaryblades and a plurality of secondary blades. The primary blades extendcloser to the bit's central axis than the secondary blades. The bit hasa set of primary nozzles associated with the primary blades and a set ofsecondary nozzles associated with the secondary blades. To generate asubstantially uniform flow through the junk slots, the primary nozzlesmay be located further inboard (i.e. closer to the central axis) andhave less profile angle and/or less (i.e. shorter) seat depth than themore radially outboard secondary nozzles.

Certain bits may also have more than two sets of nozzles with nozzleswithin a set having similar parameters that are different fromparameters of nozzles within other sets. Multiple-blade drag bits mayalso have multiple nozzles per blade or more than one nozzle in a givenjunk slot. In certain embodiments, more than one nozzle may bepositioned in a given junk slot while another junk slot has only asingle nozzle. Referring now to FIG. 5, drill bit 41 comprises blades43, junk slots 45 and 47, and nozzles 48. Two nozzles 48 are disposedwithin junk slot 47 while junk slot 45 only has one nozzle 48. Incertain embodiments, nozzles at the same radius may have substantiallythe same nozzle parameters such that the inner-most nozzles have similarparameters and outer-most nozzle have similar parameters, where the twosets not being equal.

Referring now to FIG. 6, in one embodiment, the bit design can bemodified by adjusting one or more nozzle parameters. One such nozzleparameter is the nozzle orientation, or angular orientation, which inone case may be described as comprising the skew angle 44 and profileangle 46. As the angular orientation increases, the nozzle is tiltedmore radially outward from the center of the bit body. In one example,the nozzle has a nozzle axis along the center line of the fluid flowpath from the nozzle. The nozzle also has a nozzle plane that is a planethrough the nozzle axis and parallel to the bit axis. The skew angle 44is the angle between the nozzle plane and the plane through the bit axisand the intersecting point of the nozzle axis and the bit body. Theprofile angle 46 is the angle between the nozzle axis and an axis on thenozzle plane that is collinear to the bit axis. Another nozzle parameteris nozzle location 48, which is defined as the radial, axial, andangular location of the nozzle relative to the bit's central axis. Incertain embodiments, adjusting the nozzle location of one or more of thenozzles so that at least one nozzle has a radial, axial, or angularlocation that is different than a radial, axial, or angular location ofanother nozzle may result in a substantially uniform flow. Those skilledin the art will appreciate that the invention is not limited by theexample definitions of nozzle orientation and nozzle location providedabove.

An example of a bit having adjusted nozzle parameters is shown in FIG. 7where bit 50 utilizes smaller angular orientations for nozzles 52 thatare closer to primary blades 54 so as to direct fluid at the portions ofthe blades that are closer to the bit axis. As a result, the impingementangles formed by the primary jet axis and the hole-bottom profile areclose to impingement angles of the secondary jets. Therefore, in certainembodiments, adjusting the angular orientations for one or more of thenozzles so that at least one nozzle has a angular orientation that isdifferent than a angular orientation of another nozzle may result in asubstantially uniform flow.

Another nozzle parameter that can be adjusted is seat depth, or how farthe nozzle is recessed into the bit body. Referring now to FIGS. 8-10,an increased seat depth for the nozzles further away from the bit axiscan also be used to adjust the flow distribution. FIG. 8 shows aschematic of a nozzle 60 on a bit body 62 with “standard” nozzle seatdepth 64 relative to the bit body surface 66. FIG. 9 shows a nozzle 70on a bit body 72 and having an increased seat depth 74 relative to thebit body surface 76. FIG. 10 shows a six-blade bit 80 where the primarynozzles 82, which are closer to the bit axis, have a shallower nozzleseat depth than secondary nozzles 84. Therefore, in certain embodiments,adjusting the seat depth for one or more of the nozzles so that at leastone nozzle has a seat depth that is different than a seat depth ofanother nozzle may result in a substantially uniform flow.

In one or more embodiments, smaller exit area nozzles can be used toprovide further uniformity to the flow distribution. Smaller exit areanozzles may be desired for use with those junk slots that show a largeflow rate when equal sized nozzles are used. In other embodiments, anozzle having a design that creates a large flow rate through aparticular slot could be replaced be a nozzle having the same nozzleexit area, but of somewhat hydraulically inefficient design so as toeffectively reduce the flow rate through the junk slot. Conversely, anexhibited low flow rate could be remedied by replacing the nozzle withone having a more hydraulically efficient design to increase the flow.Thus, in certain embodiments, adjusting the size or design of one ormore of the nozzles so that at least one nozzle has a size or designthat is different than a size or design of another nozzle may result ina substantially uniform flow.

It has also been found that fluid can flow in and out of a given junkslot by crossing the blade, in particular the upper portion of the bladenear the blade top. Thus, in another embodiment, a bit design parameterthat can be adjusted is blade geometry, such as the size and shape ofthe blades. For example, additional supporting material can be added tothe back of the blades or the blades can be made thicker. Not only doesthe additional material support the cutting structure, the material canalso prevent or reduce fluid flow across the top of the blades. Hence,fluid flows within each passage and junk slot with less interferencefrom cross-flow. The added blade material may protrude as close asdesired to the hole-bottom. One embodiment of a bit having adjustedblade geometry is shown in FIG. 11, where bit 100 includes blades 104having additional material 102 added to the upper portion of the blades.Alternatively, in one or more embodiments, secondary blades may extendcloser to the bit axis so as to reduce flow interference betweendifferent flow passages and increase the uniformity of flow. In otherembodiments, the blade geometries are formed such that fluid flow acrossthe top of the blades is allowed such that the flow distribution in thejunk slots is substantially uniform.

FIG. 12 shows a comparison of the flow distribution of a newconfiguration and flow distribution in the original configuration ofFIG. 1 (labeled Configuration A). The embodiment of configuration B hasadditional blade material and employs smaller angular orientation andsmaller nozzle depth for one or more nozzles. Results are obtained froma CFD analysis such as that described in FIG. 2. The standard deviationreduced from 11% to 3% for the new configuration. Thus, the newconfiguration is considered to have a more uniform flow distribution, ora balanced flow distribution. An even distribution of 16.7% in each ofthe junk slots with zero standard deviation as shown with the dash line.

Thus, embodiments of the present invention include apparatus and methodsthat allow a designer to establish a desirable flow allocation byadjusting various bit design parameters. A given drill bit design isanalyzed to determine how the flow around the bit is allocated among aplurality of flow paths. Results of the analysis are then evaluated todetermine if the flow allocation is within desirable limits. The bitdesign parameters can then be altered to adjust the flow allocation andthe bit design analyzed again. Thus, this method provides multipledesign iterations so as to determine the optimum flow allocation for aparticular drill bit design and application.

Under this design methodology, the designer, using computer-aided designsoftware and/or laboratory testing, can perform a thorough evaluation ofthe bit hydraulics prior to testing in the field, thus helping toprevent expensive downhole problems. This method may include the use ofComputational Fluid Dynamics (CFD) software to determine the flow fieldparameters around a bit and a method of evaluation to determine thefluid flow around a bit and identify manufacturer controlled bitparameters that can be adjusted to control the allocation of the fluidflow.

In certain applications, the described flow allocation methodologies canbe used to create specific situations other than uniform flow, such asbiasing the flow allocation toward one region of the bit. For example, afive bladed bit may benefit from having flow allocations of 23%, 18%,23%, 18%, and 18% instead of the uniform 20%. This non-uniformallocation of flow may be desirable in addressing an area of the bitthat has shown a need for additional cooling or cleaning.

While limited embodiments of this invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the scope or teaching of this invention. Theembodiments described herein are merely examples and are not limiting bythe type and configuration of the drill bit. Many variations andmodifications of the system and apparatus are possible and are withinthe scope of the invention. For example, the relative dimensions ofvarious parts, the materials from which the various parts are made, andother parameters can be varied, so long as the apparatus retain theadvantages discussed herein.

1. A method for designing a drill bit, comprising: modeling a domainbetween a drill bit having a first design and a surrounding wellbore;defining a plurality of regions, wherein one of the plurality of regionsis disposed within each of a plurality of flow paths through which fluidtravels through the domain; determining an allocation of flow among theplurality of flow paths through the domain; and modifying the firstdesign of the drill bit such that the allocation of flow issubstantially uniform among the plurality of flow paths.
 2. The methodof claim 1 wherein the plurality of regions are disposed within junkslots.
 3. The method of claim 1 wherein the first design is modified byadjusting a nozzle or port parameter.
 4. The method of claim 3 whereinthe nozzle or port parameter is an angular orientation, position, size,or seat depth.
 5. The method of claim 1 wherein the first design ismodified by changing the number of nozzles or ports.
 6. The method ofclaim 1 wherein the first design is modified by adjusting a bit geometryparameter.
 7. The method of claim 6 wherein the bit geometry parameteris a blade size, blade shape, or bit body geometry.
 8. The method ofclaim 1 wherein the flow allocation is determined based on a fluidicproperty of fluid moving through the plurality of regions.
 9. The methodof claim 8 wherein the fluidic property is a volumetric flow rate, massflow rate, or net flow rate.
 10. The method of claim 1 furthercomprising: modeling a modified domain between a drill bit having amodified design and the surrounding wellbore; defining a plurality ofregions within the modified domain, wherein each region is disposedwithin each of a plurality of flow paths through which fluid travelsthrough the modified domain; determining an allocation of flow among theplurality of flow paths through which fluid travels through the modifieddomain; and selecting a bit design based on a comparison of theallocation of flow among the flow paths in the modified domain and theallocation of flow among the flow paths in the unmodified domain. 11.The method of claim 1 further comprising generating a visualrepresentation of the allocation of flow among the flow paths.
 12. Adrill bit comprising: a bit body having a plurality of blades projectingthere from, wherein at least one blade has a greater length than atleast one other blade; a plurality of nozzles or ports disposed on saidbody; and a plurality of junk slots formed between adjacent blades,wherein said junk slots provide a passageway for the flow of the fluidfrom said plurality of nozzles or ports, wherein bit design parametersare such that the flow of fluid through each of the plurality of junkslots is substantially uniform.
 13. The drill bit of claim 12 whereinthe bit design parameters comprise a nozzle or port parameter or a bitgeometry parameter.
 14. The drill bit of claim 12 wherein at least oneof said plurality of nozzles or ports has nozzle or port parameter thatis different than a nozzle or port parameter of another of saidplurality of nozzles or ports.
 15. The drill bit of claim 12 wherein thenozzle or port parameter comprises at least one of, quantity, design,size, radial location, axial location, angular orientation, seat depth,and arrangement.
 16. The drill bit of claim 12 wherein at least one ofsaid junk slots has a greater number of said plurality of nozzles orports positioned there within than are positioned within another of saidplurality of junk slots.
 17. The drill bit of claim 12 wherein at leastone of said plurality of blades has more material at an upper portion ofthe blade, extends farther toward a central bit axis, or has a greaterthickness than another of said plurality of blades.
 18. The drill bit ofclaim 12 wherein said plurality of nozzles or ports comprises a numberof nozzles or ports equal to the number of said plurality of blades. 19.A drill bit of claim 12 further comprising: a central bit axis throughthe bit body; and wherein the plurality of blades extending from saidbit body, wherein said plurality of blades comprises at least oneprimary blade and at least one secondary blade, wherein at least one ofthe primary blades extends closer to the central bit axis than at leastone of the secondary blade.
 20. The drill bit of claim 19 wherein saidplurality of nozzles or ports comprises at least one primary nozzle orport and at least one secondary nozzle or port.
 21. The drill bit ofclaim 19 wherein the at least one primary nozzle or port is locatedcloser to the bit central axis than the at least one secondary nozzle orport.
 22. The drill bit of claim 19 wherein the at least one primarynozzle or port has a smaller angular orientation than the at least onesecondary nozzle or port.
 23. The drill bit of claim 19 wherein the atleast one primary nozzle or port has a shorter seat depth than the atleast one secondary nozzle or port.